Which one of the following statements is correct?

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The oxidation number for hydrogen is always zero.

The oxidation number for hydrogen is always +1.

The oxidation number for hydrogen is always -1.

Hydrogen can have more than one oxidation number.

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This question was previously asked in

UPSC CDS-2 – 2016

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Hydrogen can exist in different chemical environments where it exhibits more than one oxidation number.

The oxidation number for hydrogen is typically +1 when bonded to nonmetals (e.g., H₂O, HCl, NH₃). However, when bonded to more active metals (forming metal hydrides like NaH, CaH₂), hydrogen is more electronegative than the metal and has an oxidation number of -1. In its elemental form (H₂), the oxidation number is 0.

The oxidation number reflects the hypothetical charge an atom would have in a molecule if all bonds were ionic. The electronegativity difference between hydrogen and the atom it is bonded to determines whether hydrogen takes a positive or negative oxidation state.

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